Semiconductor device and method of manufacturing the same

ABSTRACT

A crystalline semiconductor film in which the position and the size of crystal grains are controlled is provided, and a TFT that can operate at high speed is obtained by forming a channel formation region of the TFT from the crystalline semiconductor film. A heat retaining film is formed on an insulating surface, a semiconductor film is formed to cover the heat retaining film, and a reflective film is formed to partially cover the semiconductor film. The reflective films and the semiconductor film are irradiated with a laser beam. The reflective film creates a distribution in effective irradiation intensity of laser beam on the semiconductor film. The distribution, with the heat retaining effect provided by the heat retaining film, generates a temperature gradient in the semiconductor film. Utilizing these, the position where crystal nuclei are to be generated and the direction in which crystal growth should advance can be controlled and crystal grains having a large grain size can be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing asemiconductor device having circuits that are composed of thin filmtransistors (hereinafter referred to as TFTs). Specifically, theinvention relates to the structure of electro-optical devicesrepresented by liquid crystal display devices and of electric applianceshaving as their parts the electro-optical devices, and the inventionalso relates to how to manufacture the devices. The term semiconductordevice herein refers to a device in general which utilizes semiconductorcharacteristics to function, and the electro-optical devices andelectric appliances described above fall within this category.

2. Description of the Related Art

A technique that has been a popular research subject in recent years isto use laser annealing to crystallize an amorphous semiconductor filmformed on an insulating substrate such as a glass substrate or toimprove crystallinity of a crystallized film. The amorphoussemiconductor film is often formed from silicon.

A glass substrate has advantages over a synthesized quartz glasssubstrate often used in the past, for it is inexpensive, is readilyprocessible, and easily allows a large surface area to be obtained.These are the reasons for the flood of researches mentioned above. Laserannealing is preferred in crystallizing a film on a glass substratebecause glass substrates have low melting point. A laser can give highenergy only to an amorphous semiconductor film without increasing thetemperature of the glass substrate on which the film is formed much.

A crystalline semiconductor is composed of many crystal grains and hencealso called a polycrystalline semiconductor. A semiconductor film havingcrystal grains whose grain size is larger than the grain size of crystalgrains of a semiconductor film is called a crystalline semiconductorfilm. A crystalline semiconductor film formed by laser annealing hashigh mobility. Therefore TFTs formed from crystalline semiconductorfilms are frequently used in, for example, a monolithic liquid crystalelectro-optical device in which pixel TFTs and driver circuit TFTs areformed on the same glass substrate.

An annealing method that is highly productive and industrially superiorand hence is widely employed includes: choosing a high power pulse lasersuch as an excimer laser; processing the pulse laser beam by an opticalsystem into a spot beam that forms a few centimeter square on anirradiation surface, or into a linear beam extending 10 centimeters orlonger on the irradiation surface; and performing scanning with theprocessed laser beam over the irradiation surface (or moving the laserbeam irradiation position relative to the irradiation surface).

The linear laser beam is particularly productive, for laser irradiationof the entire irradiation surface can be done by running the linear beamonly in the direction perpendicular to the longitudinal direction of thelinear beams, unlike the spot-like laser beam that has to be used forscanning in both longitudinal and lateral directions. The linear laserbeam is run in the direction perpendicular to the longitudinal directionbecause it is the most efficient scanning direction. Owing to this highproductivity, laser annealing that uses a linear beam obtained byprocessing a pulse oscillation excimer laser beam through an appropriateoptical system is becoming a mainstream technique for manufacturing aliquid crystal display device or the like from TFTs. This technique hasmade a monolithic liquid crystal display device reality in which TFTsfor forming a pixel portion (pixel TFTs) and TFTs for forming drivercircuits to be provided in the periphery of the pixel portion are formedon the same glass substrate.

However, in a crystalline semiconductor film formed by laser annealing,plural crystal grains mass, so that the crystal grains with irregulargrain sizes are distributed unevenly. In a TFT formed on a glasssubstrate, its crystalline semiconductor film is divided intoisland-like patterns in order to separate elements. With crystal grainsof irregular grain sizes distributed unevenly, it is impossible tospecify the position and the size of the crystal grains in forming aTFT. There are much more recombination centers and trap centers due tothe amorphous structure or crystal defects in the interface betweencrystal grains (crystal grain boundary) than inside the crystal grains.It is known that if carriers are trapped in these trap centers, thepotential in the crystal grain boundary is raised to block the carriersand degrade the current transportation characteristic of the carriers.While electric characteristics of a TFT heavily depend on thecrystallinity of the semiconductor film for forming a channel formationregion; it has been almost impossible to remove the adverse effects ofcrystal grain boundary and form the channel formation region from asingle crystal semiconductor film.

In order to solve those problems, various attempts have been made tocontrol the position of crystal grains and increase the grain size bylaser annealing. Now, a process a semiconductor film takes to solidifyafter the semiconductor film is irradiated with a laser beam isdescribed first.

It takes a while for the semiconductor film that has been thoroughlymelted by laser beam irradiation to form crystal nuclei. When aninfinite number of crystal nuclei are evenly (or unevenly) generated ina thoroughly melted region and grow into crystals, the solidificationprocess is completed for the thoroughly melted semiconductor film. Thecrystal grains obtained through this are distributed unevenly and haveirregular grain sizes.

If the laser beam irradiation fails to melt the semiconductor filmthoroughly and a solid phase semiconductor region partially remains,crystal growth is started immediately after the laser beam irradiationfrom the solid phase semiconductor regions. As mentioned before, ittakes a while for the thoroughly melted region to generate crystalnuclei. Therefore, until crystal nuclei are generated in the thoroughlymelted region, solid-liquid interface (meaning the border between thesolid phase semiconductor region and the thoroughly melted region) thatis the crystal growth front moves in a direction parallel to the surfaceof the semiconductor film (hereinafter referred to as lateraldirection). This causes crystal grains to grow to gain a length severaltens longer than the thickness of the semiconductor film. Such growth isended when an infinite number of crystal nuclei are evenly (or unevenly)generated and grow into crystals in the thoroughly melted region. Thisphenomenon will hereinafter be called a super lateral growth.

An amorphous semiconductor film and a polycrystalline semiconductor filmalso have a region in which the energy of the laser beam is high enoughto induce the super lateral growth. However, such high energy region isvery narrow and where a crystal grain having a large grain size is to beformed cannot be controlled. In addition, regions other than the regionin which crystal grains having a large grain size are formed aremicrocrystalline regions in which an infinite number of crystal nucleiare generated, or amorphous regions.

As described above, the position and the direction of crystal graingrowth can be controlled if the temperature gradient in the lateraldirection can be controlled (namely, if a heat flow running in thelateral direction can be generated) in the high energy region in whichthe energy of a laser beam is high enough to melt the semiconductor filmthoroughly. Achieving this control has been tackled from various angles.

For example, a method of forming crystal grains at designed positions isdescribed in “Lateral growth control in excimer laser crystallizedpolysilicon: Thin Solid Films 337 (1999), p 137-p 142). First, a metalfilm (a single layer of Cr or a laminate film obtained by layering an Alfilm on a Cr film) is formed on an amorphous semiconductor film and ispartially etched to form a metal film region and a metal film lessregion on the amorphous semiconductor film. The reflectance of Cr whenthe wavelength is 308 nm is about 60% and the reflectance of Al for thesame wavelength is about 90%. Accordingly, in irradiation of laser beamhaving a wavelength of 308 nm, an amorphous semiconductor region underthe metal film is irradiated less than an amorphous semiconductor regionthat is not covered with the metal film. In other words, there is atemperature gradient between the amorphous semiconductor region underthe metal film and the amorphous semiconductor region that is notcovered with the metal film. Therefore crystal nuclei generated in theamorphous semiconductor region under the metal film grow laterallytoward the amorphous semiconductor region that is not covered with themetal film and that remains melted. According to the report, crystalgrains having a grain size of 1 to 2 μm are formed through the lateralgrowth.

Masakiyo Matsumura of Tokyo Institute of Technology, et al. made apresentation at the forty-seventh meeting of The Japan Society ofApplied Physics and Related Societies about a method of forming acrystal grain having a large grain size at a designed position.According to the method, an organic SOG film is formed on a glasssubstrate and a silicon oxide film is formed on the organic SOG film. Onthe silicon oxide film, an amorphous silicon film is formed to bury aninsulating layer (buried insulating layer) in the amorphous silicon film(FIG. 6C). The buried insulating layer is quadrangular in top view andat least one vertex of the quadrangle is 60°.

The silicon oxide film and the glass substrate form a random network ofSi—O bonds. Accordingly, when the silicon oxide film is formed on theglass substrate and the silicon oxide film is irradiated with a laserbeam, the energy given by the laser beam irradiation is easilytransmitted to the glass substrate. However, if the silicon oxide filmhas a carbon-containing functional group (a silicon oxide film having acarbon-containing functional group is referred to as functional groupcontaining silicon oxide film in this specification), the functionalgroup terminates the bond and inhibits the film from participating informing the network of Si—O bonds. A functional group containing siliconoxide film formed on a substrate thus has low heat transmission rate,effectively working as a heat retaining film. In this specification,having a heat transmission rate lower than that of the silicon oxidefilm and of the glass substrate is equal to having a heat retainingeffect, and a film having the heat retaining effect is called a heatretaining film. A high heat transmission rate herein means a high heatconductivity whereas a low heat transmission rate means a low heatconductivity. In irradiating the silicon oxide film with a laser beam, aphase shift mask (FIG. 6A) is used to give a gradient in energy of thelaser beam (FIG. 6B). Allegedly, the method thus form crystal grainshaving a large grain size at designed positions.

An article by R. Ishihara and A. Burtsev, published in AM-LCD '98, p153-p 156, 1998, reports a laser annealing method in which a highmelting point metal film is formed between a substrate and a siliconoxide film serving as a base film, an amorphous silicon film is formedabove where the high melting point metal film is formed, and thesubstrate is irradiated with an excimer laser beam from both the frontand back (the front side of a substrate is herein defined as a surfaceon which films are formed and the back side thereof is defined as asurface opposite to the surface on which films are formed). A laser beamapplied to the front side of the substrate is absorbed by the siliconfilm and changed into heat. On the other hand, a laser applied to theback side of the substrate is absorbed by the high melting point metalfilm and changed into heat, thereby heating the high melting point metalfilm to a high temperature. The silicon oxide film provided between theheated high melting point metal film and the silicon film serves as aheat accumulating layer, so that the melted silicon film cools slowly.According to the report, a large crystal grain with the maximum diameterbeing 6.4 μm can be formed in an arbitrary place by forming the highmelting point metal film in an arbitrary place.

A method called sequential lateral solidification method (the SLSmethod) has been developed by James S. Im of Columbia University, et al.to induce super lateral growth in a desired place. In the SLS method, amask having a slit is moved along for every shot by a distancecorresponding to the length of super lateral growth (about 0.75 μm) tocrystallize the film.

The method in which a metal film is partially formed on an amorphoussemiconductor film by laser beam irradiation for crystallization hasdrawbacks. Crystal grains obtained by this method have a small grainsize of 1 to 2 μm. Also, the method can control where the crystal grainsare to be formed but it cannot control the formation position on asingle crystal basis. The metal film that is formed directly on theamorphous semiconductor film also can cause a problem, in that the metalelements in the film diffuse into the amorphous semiconductor film. Ifthis amorphous semiconductor film with the diffused metal elements iscrystallized to form a crystalline semiconductor film and thecrystalline semiconductor film is used to form a TFT, the TFT may havedegraded electric characteristics. Furthermore, the method may causecracking or peeling in the metal film and the amorphous semiconductorfilm.

In the method disclosed by Matsumura et al., the phase shift mask isnecessary to give gradient to laser beam energy. In order to positionthe phase shift mask relative to the buried insulating layer, controlwith a micron-level precision is needed to thereby make the laserirradiation apparatus for this method more complicated than an ordinarylaser irradiation apparatus. Also, the buried insulating layer that isquadrangular in top view with one or more corners of the quadranglehaving an angle as wide as 60° results in too many crystal nuclei in thesemiconductor film below the wide corner or corners when thesemiconductor film that has been melted by laser irradiation is cooleddown. These crystal grains crowd the film and collide with one anotheras they grow, thereby lowering possibility of obtaining crystal grainsof large grain size. Furthermore, the complicate structure of burying aninsulating layer in an amorphous semiconductor film cause the troublewhen it comes to forming a TFT. The trouble is that the buriedinsulating layer remains despite the fact that it has nothing to do withthe actual function of the TFT.

The method proposed by R. Ishihara et al. can form a semiconductor filmthat may be used as an active layer of a top gate TFT structurally.However, this top gate TFT will have difficulty in operating at highspeed because the silicon oxide film provided between the amorphoussemiconductor film and the high melting point metal film generatesparasitic capacitance to increase current consumption. On the otherhand, the method will be useful for a bottom gate TFT or a reversedstagger TFT, for the high melting point metal film can serve as a gateelectrode. Still, the method requires that a silicon oxide film isformed on a substrate, a high melting point metal film is formed on thesilicon oxide film, and an amorphous silicon film is formed above wherethe high melting point metal film is formed. The thickness thereof, evenif not counting the thickness of the semiconductor film in andconsidering only the thickness of the high melting point metal film andthe silicon oxide film, does not amount to a thickness that is suitableboth for crystallization process and for a TFT element in terms ofelectric characteristics. Thus the method cannot satisfy the optimaldesign for crystallization process and the optimal design for elementstructure simultaneously.

Moreover, when a high melting point metal film that does not transmitlight is formed over the entire-surface of a glass substrate, it cannotform a transmissive liquid crystal display device. Also, a chromium (Cr)film or a titanium (Ti) film used as the high melting point metal filmhas high internal stress, which probably leads to insufficient adhesionto the glass substrate. The high internal stress also influences thesemiconductor film to be formed above the high melting point metal filmand is likely to cause distortion in the resultant crystallinesemiconductor film.

On the other hand, in order to control the threshold voltage(hereinafter referred to as Vth) that is an important parameter in TFTsso that it falls within a given range, charged electrons in a channelformation region has to be controlled. In addition, in order to obtaincontrolled Vth, it is required that charge defect density is reduced ina base film formed from an insulating film in contact with an activelayer as well as in a gate insulating film and that the internal stressin the films is balanced. These requirements are suitably met by amaterial containing silicon as its ingredient, such as a silicon oxidefilm or a silicon oxynitride film. Therefore, there is a fear that thehigh melting point metal film provided between the substrate and thebase film will disturb the balance.

The SLS method requires control with a micron-level precision inpositioning the mask relative to the substrate, thereby making the laserirradiation apparatus for this method more complicated than an ordinaryone. Moreover, the method has a problem in throughput when it is used toform a TFT for a liquid crystal display device having a large arearegion.

SUMMARY OF THE INVENTION

The present invention has been made to solve those problems and anobject of the present invention is therefore to provide a crystallinesemiconductor film in which the position and the size of crystal grainsare controlled, and to provide a TFT that can operate at high speed byforming a channel formation region of the TFT from the obtainedcrystalline semiconductor film. Another object of the present inventionis to provide a technique of applying the obtained TFT to varioussemiconductor devices such as a transmissive liquid crystal displaydevice and a display device that uses an electro-luminescence material.

The present invention increases the grain size of crystal grains of acrystalline semiconductor film formed by laser annealing. The inventionis characterized in that the cooling process of a semiconductor film isslowed down using a heat retaining film formed between the semiconductorfilm and a substrate to reduce the heat loss rate of heat given by laserbeam irradiation and that a reflective film is formed on a region of thesemiconductor film which does not overlap the heat retaining film tocreate a temperature gradient in the semiconductor film, whereby crystalgrains having a large grain size are formed at designed positions. Thereflective film in this specification refers to a film having highreflectance. Since crystal growth length is in proportion to the productof growth time (a time period a melted semiconductor film takes tosolidify) and growth rate (speed at which the solid-liquid interfacemoves), the grain size is increased as the cooling rate of thesemiconductor film is lowered to prolong the growth time. The positionof the crystal grain can also be controlled by controlling the coolingrate.

The heat retaining film is formed using a silicon oxide film thatcontains methyl (CH₃), ethyl (C₂H₅), propyl (C₃H₇), butyl (C₄H₉), vinyl(C₂H₃), phenyl (C₆H₅), or CF₃ group (functional group containing siliconoxide film). A silicon oxide film containing any one of the groups givenabove does not participate in forming a network of Si—O bonds becausethe functional group terminates the bonds. The heat transmission rate isthus lowered and the film works effectively as the heat retaining film.It is also effective to use a porous silicon film or a porous siliconoxide film to form the heat retaining film. Owing to the pores, heattransmission rate is low in a porous silicon film or a porous siliconoxide film to make the film useful as the heat retaining film.

When the functional group containing silicon oxide film is used for theheat retaining film, it is desirable to form an insulating film on thefunctional group containing silicon oxide film in order to preventdiffusion of impurities from the functional group containing siliconoxide film. In the case of using a porous silicon film or a poroussilicon oxide film for the heat retaining film also, forming aninsulating film on the porous silicon film or the porous silicon oxidefilm is desirable in order to keep the surface level for the poroussilicon film or the porous silicon oxide film.

Described next is a method of varying the effective irradiationintensity of a laser beam on the semiconductor film by forming thereflective film to partially cover the semiconductor film. Adistribution in effective irradiation intensity of laser beam can becreated if a laser beam irradiates the semiconductor film from the sidewhere the reflective film partially covers the semiconductor film. Thedescription here takes as an example the case where the reflective filmis a metal film and the semiconductor film is an amorphous silicon film.

When an amorphous film with a thickness of 55 nm is irradiated with alaser beam, the reflectance varies depending on the wavelength of thelaser beam as shown in FIG. 4. When a metal film is irradiated with alaser beam also, the reflectance varies depending on the wavelength ofthe laser beam as shown in FIG. 5. In order to create a distribution ineffective irradiation intensity of laser beam on the semiconductor filmby forming a reflective film so as to partially cover the semiconductorfilm, the reflectance against the reflective film and the reflectanceagainst the semiconductor film have to be the same, at least.Preferably, the reflectance against the reflective films is higher thanthe reflectance against the semiconductor film. However, note that theoptimal condition may differ from mode to mode because the reflectancevaries depending on the wavelength of the laser beam, the kind and thethickness of the semiconductor film, the kind of the reflective film,and the like.

When the distribution in effective energy irradiation intensity iscreated by this method on the semiconductor film and the reflectivefilm, a region of the semiconductor film which is under the reflectivefilm receives laser beam irradiation of reduced intensity and does notmelt thoroughly. As has been mentioned, if a solid phase semiconductorregion remains partially, crystal growth is started immediately afterthe laser beam irradiation from the solid phase semiconductor region.The solid-liquid interface that is the crystal growth front moves in thelateral direction until crystal nuclei are generated in a thoroughlymelted region. Crystal grains grow in this way and hence the obtainedcrystal grains can have a large grain size. Even if the region of thesemiconductor film which is under the reflective film is thoroughlymelted, the irradiation intensity of laser beam is not as strong as theintensity in the region of the semiconductor film which is not coveredwith the reflective film, and the region of the semiconductor film whichis under the reflective film cools faster than the other region of thesemiconductor film. Therefore crystal growth is started from the regionof the semiconductor film which is under the reflective film and thecrystal grains grow toward the other region of the semiconductor film.However, impurities will probably diffuse into the semiconductor filmfrom the reflective film and cracking and peeling are likely to takeplace in the semiconductor film and the reflective film if thereflective film is formed directly on the semiconductor film. It istherefore desirable to form an insulating film between the semiconductorfilm and the reflective film.

The reflective film is polygonal in top view, and this polygon has anangle which is smaller than 60°. With the reflective film shaped assuch, the crystal nuclei are generated at a smaller density in thesemiconductor film below the vertex when the semiconductor film isirradiated with the laser beam. Thus collision between growing crystalgrains can be avoided.

The insulating film formed between the semiconductor film and thereflective film can function also as a reflection preventive film. FIGS.3A and 3B show changes in reflectance in the case where a silicon oxidefilm is formed on an amorphous silicon film (having a thickness of 55nm) and a laser beam irradiates the films from the silicon oxide filmside with the thickness of the silicon oxide film as the parameter. Inthe case of FIG. 3A, the wavelength of the laser beam for irradiation is308 nm whereas a laser beam having a wavelength of 532 nm is used forthe irradiation in the case of FIG. 3B. It can be seen in FIG. 3A thatthe reflectance changes periodically and that the silicon oxide film canfunction as the reflection preventive film if its thickness is that whenthe reflectance is low in the graphs. The insulating film is not limitedto a silicon oxide film and other insulating films can function as thereflection preventive film, of course.

The substrate may be heated up to about 500° C. before the laser beamirradiation. Expectedly, this will lower the heat loss rate in thesemiconductor film to increase the grain size of the crystal grains.

Thus the crystalline semiconductor film obtained through laser annealingby forming a heat retaining film between a semiconductor film and asubstrate and forming a reflective film in a region of the semiconductorfilm which does not overlap the heat retaining film can be applied tovarious semiconductor devices. The crystalline semiconductor film isparticularly suitable to form an active layer of a TFT.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D are diagrams showing an example of a method of formingcrystal grains having a large grain size at designed positions inaccordance with the present invention;

FIGS. 2A to 2C are diagrams showing the example of the method of formingcrystal grains having a large grain size at designed positions inaccordance with the present invention;

FIGS. 3A and 3B are graphs showing the reflectance against a siliconoxide film in laser beam irradiation with the thickness of the siliconoxide film as the parameter, where FIG. 3A uses a laser beam having awavelength of 308 nm for the irradiation and FIG. 3B uses a laser beamhaving a wavelength of 532 nm for the irradiation;

FIG. 4 is a graph showing the relation between the wavelength and thereflectance against an amorphous silicon film with a thickness of 55 nm;

FIG. 5 is a graph showing the relation between the wavelength and thereflectance against metals;

FIG. 6A is a diagram showing an example of a phase shift mask, FIG. 6Bis a graph showing a distribution in intensity of laser beam after thebeam passes through the phase shift mask, and FIG. 6C is a diagramshowing an example of conventional methods for forming crystal grainshaving a large grain size at designed positions;

FIGS. 7A and 7B are diagrams showing an example of the method of formingcrystal grains having a large grain size at designed positions inaccordance with the present invention;

FIGS. 8A and 8B are diagrams showing another example of the method offorming crystal grains having a large grain size at designed positionsin accordance with the present invention;

FIGS. 9A to 9C are cross sectional views showing a process ofmanufacturing a pixel TFT and a driver circuit TFT;

FIGS. 10A to 10C are cross sectional views showing the process ofmanufacturing a pixel TFT and a driver circuit TFT;

FIG. 11 is a cross sectional view showing the process of manufacturing apixel TFT and a driver circuit TFT;

FIG. 12 is a top view showing the structure of a pixel TFT;

FIG. 13 is a cross sectional view showing a process of manufacturing anactive matrix liquid crystal display device;

FIGS. 14A to 14F are diagrams showing examples of a semiconductordevice;

FIGS. 15A to 15D are diagrams showing examples of the semiconductordevice; and

FIGS. 16A to 16C are diagrams showing examples of the semiconductordevice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment mode of the present invention will be described withreference to cross sectional views of FIGS. 1A to 2C. In FIGS. 1D and2C, top views as well as sectional views are shown.

A base insulating film 12 is formed on a substrate 11 by a known method(LPCVD, plasma CVD, or the like) from a silicon nitride film, a siliconoxynitride film, a silicon oxide film or a like other film.

A heat retaining film 13 is formed on the base insulating film 12. Theheat retaining film is a silicon oxide film that contains methyl (CH₃),ethyl (C₂H₅), propyl (C₃H₇), butyl (C₄H₉), vinyl (C₂H₃), phenyl (C₆H₅),or CF₃ group (functional group containing silicon oxide film).Alternatively, a porous silicon film or a porous silicon oxide film isused to form the heat retaining film.

Considering the heat conductivity of the substrate (1.4 W/m·k, in thecase of a quartz substrate) and the heat conductivity of silicon oxide(1 to 2 W/m·k), the heat retaining film 13 desirably has a heatconductivity of 1.0 W/m·k or less, more desirably 0.3 W/m·k or less.

After the heat retaining film 13 is formed, photolithography is used toform a resist mask and to etch unnecessary portions of the heatretaining film 13 away. A heat retaining film 14 is thus formed.

If the heat retaining film 14 is the functional group containing siliconoxide film, it is desirable to form a first insulating film 15 in orderto prevent impurities in the heat retaining film 14 from diffusing intoa semiconductor film to be formed later. The first insulating film 15 isan insulating film, typically a silicon nitride film, a siliconoxynitride film or a silicon oxide film, formed by a known method(LPCVD, plasma CVD or the like). The first insulating film 15 is formedusing a silicon nitride film, a silicon oxynitride film, a silicon oxidefilm or the like by a known method also when the heat retaining film 14is a porous silicon film or a porous silicon oxide film. This is becausethe porous silicon film or the porous silicon oxide film has about 10¹¹pores per centimeters square on its surface and the heat retaining film14 should have a level surface.

The first insulating film 15 is etched to remove unnecessary portionsusing a resist mask by photolithography. A first insulating film 16 isformed as a result.

Next, a semiconductor film 17 is formed by a known method such as plasmaCVD or sputtering to a thickness of 10 to 200 nm (preferably 30 to 100nm). The semiconductor film 17 may be an amorphous semiconductor film, amicrocrystalline semiconductor film or a polycrystalline semiconductorfilm. A compound semiconductor film having an amorphous structure, suchas an amorphous silicon germanium film, can also be used.

In order to prevent impurities in a reflective film to be formed laterfrom diffusing into the semiconductor film, a second insulating film 18is desirably formed on the semiconductor film 17. If the secondinsulating film 18 is to function simultaneously as a reflectionpreventive film, the second insulating film has to have a thickness thatlowers the reflectance. Such thickness varies depending on thewavelength of the laser beam as shown in FIGS. 3A and 3B. The secondinsulating film 18 is formed using a silicon nitride film, a siliconoxynitride film, a silicon oxide film or a like other film by a knownmethod (LPCVD, plasma CVD or the like).

On the second insulating film 18, a reflective film 19 is formed. If thereflective film 19 is a metal film, the film is formed by a known methodsuch as sputtering or evaporation to a thickness of 10 to 200 nm(preferably 10 to 100 nm). The metal film may be formed of an elementselected from the group consisting of Ta, W, Ti, Mo, Al, Cu, Cr and Nd,or of an alloy material, or compound material, containing the aboveelements as its main ingredient. An Ag—Pd—Cu alloy may also be used.

After forming the reflective film 19, a resist mask is formed andunnecessary portions of the reflective film 19 is etched away byphotolithography. A reflective film 20 is thus formed. The shape of thereflective film 20 is not particularly limited but desirably ispolygonal in top view with one or more angles of the polygon beingsmaller than 60°. The angle smaller than 60° will hereinafter be calleda vertex A. With the reflective film shaped as such, crystal nuclei aregenerated at a smaller density in the semiconductor film below a regionaround the vertex A while the semiconductor film that has beenirradiated with a laser beam cools down. Thus collision between growingcrystal grains can be avoided.

If the second insulating film 18 deos not function as the reflectionpreventive film, photolithography is then used so that a resist mask isformed, and the region of the second insulating film which does notoverlap the reflective film is etched away. A second insulating film 21is obtained as a result.

FIG. 2A is a diagram illustrating a crystallization step in which thesubstrate is irradiated with a laser beam from the front side. Incrystallization by laser annealing, hydrogen contained in thesemiconductor film is desirably released before the annealing.Appropriately, the semiconductor film is exposed in nitrogen atmosphereat 400 to 500° C. for about an hour to reduce the hydrogen content to 5atom % or less. This improves the resistance of the film against laserremarkably.

A description is given of a laser oscillator used in laser annealing. Anexcimer laser is high power and currently can generate a high frequencypulse on the order of 300 Hz, and hence it is often used in laserannealing. Other than the pulse oscillation excimer laser, a continuouswave excimer laser, an Ar laser, a YAG laser, a YVO₄ laser, a YLF laser,etc. may be used. The laser beam irradiation can be carried out invacuum, atmospheric air, nitrogen atmosphere, or other types ofatmospheres. The substrate may be heated up to about 500° C. before thelaser beam irradiation. This will lower the heat loss rate in thesemiconductor film to increase the grain size of the crystal grains.

One of the laser oscillators listed above is chosen to irradiate thesubstrate from the front side in one of the above atmospheres, wherebythe semiconductor film is crystallized.

Here, setting the ends of the reflective film as the borders, a regionincluding the heat retaining film 14 is designated as a region A, aregion including the reflective film 20 is designated as a region B, anda region that does not include the heat retaining film 14 nor thereflective film 20 is designated as a region C. (See FIGS. 2B and 2C.)

When irradiated with a laser beam, the semiconductor film is melted.However, the effective irradiation intensity of laser beam on thesemiconductor film in the region B is lower than on the semiconductorfilm in the region A and the region C because the semiconductor film inthe region B is covered with the reflective film, which reflects thelaser beam. Accordingly a solid phase semiconductor region 23 is leftbelow the reflective film, and crystal growth begins immediately afterthe laser beam irradiation from the solid phase semiconductor region 23following the temperature gradient created in the semiconductor film.The density of generated crystal nuclei 24 is particularly low in thesolid phase semiconductor region 23 in the vicinity of the vertex A, forthe vertex A has a small angle of less than 60°. Furthermore, thesemiconductor film remains melted for a long time in the region A due tothe presence of the heat retaining film 14. Therefore the crystal nuclei24 grow toward the region A. Thus crystal grains having a large grainsize are formed in the semiconductor film in the region A. In the regionC, the semiconductor film does not have the heat retaining film 14underneath and hence cools faster than in the region A to generatecrystal nuclei and start the crystal growth. In this way, a crystallinesemiconductor film 25 is formed in which crystal grains have a grainsize larger than that of the semiconductor film before the laser beamirradiation.

The crystalline semiconductor film 25 formed by being irradiated with alaser beam is heated at 300 to 450° C. in an atmosphere containing 3 to100% of hydrogen, or heated at 200 to 450° C. in an atmospherecontaining hydrogen that is generated by plasma. The heat treatmentreduces the remaining defects.

The reflective film is then removed by photolithography or othermethods, and then the second insulating film (18 or 21) is removed byphotolithography or other methods.

The crystalline semiconductor film 25 formed in this way has a region 26in which crystal grains having a large grain size are formed as shown inthe top view of FIG. 2C. If the region 26 is used for a channelformation region of a TFT, the obtained TFT can have improved electriccharacteristics.

Embodiment 1

Embodiment 1 of the present invention will be described with referenceto cross sectional views of FIGS. 1A to 2C. In FIGS. 1D and 2C, topviews as well as cross sectional views are shown.

In FIG. 1A, a substrate is denoted by 11. The substrate 11 may be aglass substrate. Examples of the glass substrate include a synthesizedquartz glass substrate, and a non-alkaline glass substrate such as abarium borosilicate substrate or an aluminoborosilicate glass substrate.Transparent films such as PC (polycarbonate), PAr (polyarylate), PES(polyether sulfon) and PET (polyethylene telephthalate) may be usedinstead. For example, Corning No. 7059 glass or No. 1737 glass (productof Corning Incorporated.) is a preferable material for the substrate 11.

A base insulating film 12 is formed on the substrate 11 by a knownmethod (LPCVD, plasma CVD, or the like) from a silicon nitride film, asilicon oxynitride film, a silicon oxide film or a like other film. Inthis embodiment, a silicon oxynitride film (composition ratio: Si=32%,O=27%, N=24%, H=17%) is formed to a thickness of 50 nm.

On the base insulating film 12, a heat retaining film 13 is formed usinga functional group containing silicon oxide film. A description is givenon a method of forming the heat retaining film 13 from a silicon oxidefilm that contains methyl (CH₃), ethyl (C₂H₅), propyl (C₃H₇), butyl(C₄H₉), vinyl (C₂H₃), phenyl (C₆H₅), or CF₃ group. The film is formed bya vapor phase method or a liquid phase method, depending on the organicmaterial used as the row material of the film. A desirable thickness ofthe heat retaining film 13 is 100 nm to 1000 nm (more desirably 200 to500 nm). By optimizing the thickness of the heat retaining film, thecooling rate of a semiconductor film in a laser annealing step iscontrolled. If the heat retaining film is thinner than 100 nm, the filmcannot provide sufficient heat retaining effect. On the other hand, ifthe heat retaining film is thicker than 1000 nm, it causes cracking(fissure) in the semiconductor film to be formed later and hence is notdesirable In this embodiment, a methyl (CH₃) containing silicon oxidefilm is formed to a thickness of 50 nm.

After the heat retaining film 13 is formed, photolithography is used toform a resist mask and to etch unnecessary portions of the heatretaining film 13 away. A heat retaining film 14 is thus formed. Inetching the heat retaining film 13, dry etching that uses fluorine-basedgas or wet etching that uses a fluorine-based solution may be employed.When the wet etching is chosen, for example, the etchant may be amixture of 7.13% of ammonium hydrogen fluoride (NH₄HF₂) and 15.4% ofammonium fluoride (NH₄F). (The mixture is commercially available by thetrade name of LAL500 from Stella Chemipha Inc.)

Subsequently, a first insulating film 15 is formed in order to preventimpurities in the heat retaining film 14 from diffusing into thesemiconductor film to be formed later. The first insulating film 15 maybe a silicon nitride film, a silicon oxynitride film or a silicon oxidefilm formed by a known method (LPCVD, plasma CVD or the like).

The first insulating film 15 is etched to remove unnecessary portionsusing a resist mask and photolithography. A first insulating film 16 isformed as a result. In etching the first insulating film 15, dry etchingthat uses fluorine-based gas or wet etching that uses a fluorine-basedsolution may be employed. When the wet etching is chosen, for example,the etchant may be a mixture of 7.13% of ammonium hydrogen fluoride(NH₄HF₂) and 15.4% of ammonium fluoride (NH₄F). (The mixture iscommercially available by the trade name of LAL500 from Stella ChemiphaInc.)

Next, a semiconductor film 17 is formed by a known method such as plasmaCVD or sputtering to a thickness of 10 to 200 nm (preferably 30 to 100nm). The semiconductor film 17 may be an amorphous semiconductor film, amicrocrystalline semiconductor film or a polycrystalline semiconductorfilm. A compound semiconductor film having an amorphous structure, suchas an amorphous silicon germanium film, can also be used. In thisembodiment, an amorphous silicon film with a thickness of 55 nm isformed by plasma CVD.

In order to prevent impurities in a reflective film to be formed laterfrom diffusing into the semiconductor film, a second insulating film 18is formed on the semiconductor film 17. The second insulating film 18 isformed using a silicon nitride film, a silicon oxynitride film, asilicon oxide film or a like other film by a known method (LPCVD, plasmaCVD or the like). In this embodiment, a silicon oxynitride film(composition ratio: Si=32%, O=27%, N=24%, H=17%) is formed to athickness of 50 nm.

On the second insulating film 18, a reflective film 19 is formed. If thereflective film 19 is a metal film, the film is formed by a known methodsuch as sputtering or evaporation to a thickness of 10 to 200 nm(preferably 10 to 100 nm). The metal film may be formed of an elementselected from the group consisting of Ta, W, Ti, Mo, Al, Cu, Cr and Nd,or of an alloy material, or compound material, containing the aboveelements as its main ingredient. An Ag—Pd—Cu alloy may also be used. Inthis embodiment, a Cr film with a thickness of 50 nm is formed.

After forming the reflective film 19, a resist mask is formed andunnecessary portions of the reflective film 19 is etched away byphotolithography. A reflective film 20 is thus formed. The shape of thereflective film 20 is not particularly limited but desirably ispolygonal in top view with one or more angles of the polygon beingsmaller than 60°. The angle smaller than 60°will hereinafter be called avertex A. With the reflective film shaped as such, crystal nuclei aregenerated at a smaller density in the semiconductor film below a regionaround the vertex A while the semiconductor film that has beenirradiated with a laser beam cools down. Thus collision between growingcrystal grains can be avoided.

Then photolithography is used to form a resist mask on the secondinsulating film 18 and etch away the region of the second insulatingfilm which does not overlap the reflective film. A second insulatingfilm 21 is obtained as a result.

FIG. 2A is a diagram illustrating a crystallization step in which thesubstrate is irradiated with a laser beam from the front side. Incrystallization by laser annealing, hydrogen contained in thesemiconductor film is desirably released before the annealingAppropriately, the semiconductor film is exposed in nitrogen atmosphereat 400 to 500° C. for about an hour to reduce the hydrogen content to 5atom % or less. This improves the resistance of the film against laserremarkably.

A description is given of a laser oscillator used in laser annealing. Anexcimer laser is high power and currently can generate a high frequencypulse on the order of 300 Hz, and hence it is often used in laserannealing. Other than the pulse oscillation excimer laser, a continuouswave excimer laser, an Ar laser, a YAG laser, a YVO₄ laser, a YLF laser,etc. may be used. The laser beam irradiation can be carried out invacuum, atmospheric air, nitrogen atmosphere, or other types ofatmospheres. The substrate may be heated up to about 500° C. before thelaser beam irradiation. This will lower the heat loss rate in thesemiconductor film to increase the grain size of the crystal grains.

To crystallize the semiconductor film, a pulse oscillation XeCl excimerlaser oscillator is used in this embodiment and the substrate isirradiated with its laser beam from the front side in atmospheric air.

Here, setting the ends of the reflective film as the borders, a regionincluding the heat retaining film 14 is designated as a region A, aregion including the reflective film 20 is designated as a region B, anda region that does not include the heat retaining film 14 nor thereflective film 20 is designated as a region C. (See FIGS. 2B and 2C.)

When irradiated with a laser beam, the semiconductor film is melted.However, the effective irradiation intensity of laser beam on thesemiconductor film in the region B is lower than on the semiconductorfilm in the region A and the region C because the semiconductor film inthe region B is covered with the reflective film, which reflects thelaser beam. Accordingly a solid phase semiconductor region 23 is leftbelow the reflective film, and crystal growth begins immediately afterthe laser beam irradiation from the solid phase semiconductor region 23following the temperature gradient created in the semiconductor film.The density of generated crystal nuclei 24 is particularly low in thesolid phase semiconductor region 23 in the vicinity of the vertex A, forthe vertex A has a small angle of less than 60°. Furthermore, thesemiconductor film remains melted for a long time in the region A due tothe presence of the heat retaining film 14. Therefore the crystal nuclei24 grow toward the region A. Thus crystal grains having a large grainsize are formed in the semiconductor film in the region A. In the regionC, the semiconductor film does not have the heat retaining film 14underneath and hence cools faster than in the region A to generatecrystal nuclei and start the crystal growth.

Thus a crystalline semiconductor film 25 is formed by the laser beamirradiation. The crystalline semiconductor film 25 is heated at 300 to450° C. in an atmosphere containing 3 to 100% of hydrogen, or heated at200 to 450° C. in an atmosphere containing hydrogen that is generated byplasma. The heat treatment reduces the remaining defects.

The reflective film is then removed by photolithography or othermethods, and then the second insulating film is removed byphotolithography or other methods.

The crystalline semiconductor film 25 formed in this way has a region 26in which crystal grains having a large grain size are formed as shown inthe top view of FIG. 2C. If the region 26 is used for a channelformation region of a TFT, the obtained TFT can have improved electriccharacteristics.

Embodiment 2

This embodiment shows an example of forming a crystalline semiconductorfilm by a method different from the one described in Embodiment 1. Theonly difference between this embodiment and Embodiment 1 is in the stepof forming the heat retaining film 13 and the subsequent steps areidentical. Therefore explanations for the identical steps are omittedhere.

First, a substrate is prepared as in Embodiment 1. The substrate,denoted by 11, may be a glass substrate. Examples of the glass substrateinclude a synthesized quartz glass substrate, and a non-alkaline glasssubstrate such as a barium borosilicate substrate or analuminoborosilicate glass substrate. Transparent films such as PC(polycarbonate), PAr (polyarylate), PES (polyether sulfon) and PET(polyethylene telephthalate) may be used instead. For example, CorningNo. 7059 glass or No. 1737 glass (product of Corning Incorporated) is apreferable material for the substrate 11.

A base insulating film 12 is formed on the substrate 11 by a knownmethod (LPCVD, plasma CVD, or the like) from a silicon nitride film, asilicon oxynitride film, a silicon oxide film or a like other film. Inthis embodiment, a silicon oxynitride film (composition ratio: Si=32%,O=27%, N=24%, H=17%) is formed to a thickness of 50 nm.

Now, a description is given on the method of forming the heat retainingfilm 13 on the base insulating film 12 from a silicon oxide film thatcontains methyl (CH₃), ethyl (C₂H₅), propyl (C₃H₇), butyl (C₄H₉), vinyl(C₂H₃), phenyl (C₆H₅), or CF₃ group. An example of the method of formingthe heat retaining film includes generating glow discharge with amixture of TEOS and O₂, at a reaction pressure of 20 to 100 Pa, asubstrate temperature of 200 to 350° C., a high frequency of 13.56 MHz,and a power density of 0.1 to 0.5 W/cm². Though the optimal conditionsdepend on the characteristic of the apparatus, the substrate temperatureand the electric power density are usually set low. The low temperatureand density leave unbroken C_(X)H_(Y) bonds, whereby a functional groupcontaining silicon oxide film is formed. In this embodiment, a methylcontaining silicon oxide film is formed to a thickness of 50 nm.

Considering the heat conductivity of the substrate (1.4 W/m·k, in thecase of a quartz substrate) and the heat conductivity of a silicon oxidefilm (1 to 2 W/m·k), the heat retaining film 13 desirably has a heatconductivity of 1.0 W/m·k or less, more desirably 0.3 W/m·k or less.

After the heat retaining film 13 is formed, photolithography is used toform a resist mask and to etch unnecessary portions of the heatretaining film 13 away. A heat retaining film 14 is thus formed. Inetching the heat retaining film 13, dry etching that uses fluorine-basedgas or wet etching that uses a fluorine-based solution may be employed.When the wet etching is chosen, for example, the etchant may be amixture of 7.13% of ammonium hydrogen fluoride (NH₄HF₂) and 15.4% ofammonium fluoride (NH₄F). (The mixture is commercially available by thetrade name of LAL500 from Stella Chemipha Inc.)

The subsequent steps follow the corresponding steps of Embodiment 1 toobtain a crystalline semiconductor film shown in FIG. 2C. The obtainedcrystalline semiconductor film has a region 26 in which crystal grainshaving a large grain size are formed as shown in the top view of FIG.2C. If the region 26 is used for a channel formation region of a TFT,the obtained TFT can have improved electric characteristics.

Embodiment 3

This embodiment shows an example of forming a crystalline semiconductorfilm by a method different from the ones described in Embodiments 1 and2. The only difference between this embodiment and Embodiment 1 is inthe step of forming the heat retaining film 13 and the subsequent stepsare identical. Therefore explanations for the identical steps areomitted here.

First, a substrate is prepared as in Embodiment 1. The substrate,denoted by 11, may be a glass substrate. Examples of the glass substrateinclude a synthesized quartz glass substrate, and a non-alkaline glasssubstrate such as a barium borosilicate substrate or analuminoborosilicate glass substrate. Transparent films such as PC(polycarbonate), PAr (polyarylate), PES (polyether sulfon) and PET(polyethylene telephthalate) may be used instead. For example, CorningNo. 7059 glass or No. 1737 glass (product of Corning Incorporated) is apreferable material for the substrate 11.

A base insulating film 12 is formed on the substrate 11 by a knownmethod (LPCVD, plasma CVD, or the like) from a silicon nitride film, asilicon oxynitride film, a silicon oxide film or a like other film. Inthis embodiment, a silicon oxynitride film (composition ratio: Si=32%,O=27%, N=24%, H=17%) is formed to a thickness of 50 nm.

On the base insulating film 12, a silicon oxide film containing phenylis formed as the heat retaining film 13. This silicon oxide film isformed by, for example, depositing a mixture gas ofphenyltrichlorosilane (PhSiCl₃) and water (H₂O) directly on thesubstrate that has been heated up to 60 to 100° C. In this embodiment,the phenyl containing silicon oxide film has a thickness of 50 nm.

Considering the heat conductivity of the substrate (1.4 W/m·k, in thecase of a quartz substrate) and the heat conductivity of silicon oxide(1 to 2 W/m·k), the heat retaining film 13 desirably has a heatconductivity of 1.0 W/m·k or less, more desirably 0.3 W/m·k or less.

After the heat retaining film 13 is formed, photolithography is used toform a resist mask and to etch unnecessary portions of the heatretaining film 13 away. A heat retaining film 14 is thus formed. Inetching the heat retaining film 13, dry etching that uses fluorine-basedgas or wet etching that uses a fluorine-based solution may be employed.When the wet etching is chosen, for example, the etchant may be amixture of 7.13% of ammonium hydrogen fluoride (NH₄HF₂) and 15.4% ofammonium fluoride (NH₄F). (The mixture is commercially available by thetrade name of LAL500 from Stella Chemipha Inc.)

The subsequent steps follow the corresponding steps of Embodiment 1 toobtain a crystalline semiconductor film shown in FIG. 2C. The obtainedcrystalline semiconductor film has a region 26 in which crystal grainshaving a large grain size are formed as shown in the top view of FIG.2C. If the region 26 is used for a channel formation region of a TFT,the obtained TFT can have improved electric characteristics.

Embodiment 4

This embodiment shows an example of forming a crystalline semiconductorfilm by a method different from the ones described in Embodiments 1through 3. The only difference between this embodiment and Embodiment 1is in the step of forming the heat retaining film 13 and the subsequentsteps are identical. Therefore explanations for the identical steps areomitted here.

First, a substrate is prepared as in Embodiment 1. The substrate,denoted by 11, may be a glass substrate. Examples of the glass substrateinclude a synthesized quartz glass substrate, and a non-alkaline glasssubstrate such as a barium borosilicate substrate or analuminoborosilicate glass substrate. Transparent films such as PC(polycarbonate), PAr (polyarylate), PES (polyether sulfon) and PET(polyethylene telephthalate) may be used instead. For example, CorningNo. 7059 glass or No. 1737 glass (product of Corning Incorporated) is apreferable material for the substrate 11.

A base insulating film 12 is formed on the substrate 11 by a knownmethod (LPCVD, plasma CVD, or the like) from a silicon nitride film, asilicon oxynitride film, a silicon oxide film or a like other film. Inthis embodiment, a silicon oxynitride film (composition ratio: Si=32%,O=27%, N=24%, H=17%) is formed to a thickness of 50 nm.

On the base insulating film 12, a silicon oxide film containing CF₃group is formed as the heat retaining film 13. This silicon oxide filmis formed by, for example, depositing a mixture gas of CF₃Si(CH₃)₃ andozone (O₃) on the substrate that has been heated up to 300 to 400° C. Inthis embodiment, the silicon oxide film containing CF₃ group has athickness of 50 nm.

Considering the heat conductivity of the substrate (1.4 W/m·k, in thecase of a quartz substrate) and the heat conductivity of silicon oxide(1 to 2 W/m·k), the heat retaining film 13 desirably has a heatconductivity of 1.0 W/m·k or less, more desirably 0.3 W/m·k or less.

After the heat retaining film 13 is formed, photolithography is used toform a resist mask and to etch unnecessary portions of the heatretaining film 13 away. A heat retaining film 14 is thus formed. Inetching the heat retaining film 13, dry etching that uses fluorine-basedgas or wet etching that uses a fluorine-based solution may be employed.When the wet etching is chosen, for example, the etchant may be amixture of 7.13% of ammonium hydrogen fluoride (NH₄HF₂) and 15.4% ofammonium fluoride (NH₄F). (The mixture is commercially available by thetrade name of LAL500 from Stella Chemipha Inc.)

The subsequent steps follow the corresponding steps of Embodiment 1 toobtain a crystalline semiconductor film shown in FIG. 2C. The obtainedcrystalline semiconductor film has a region 26 in which crystal grainshaving a large grain size are formed as shown in the top view of FIG.2C. If the region 26 is used for a channel formation region of a TFT,the obtained TFT can have improved electric characteristics.

Embodiment 5

This embodiment shows an example of forming a crystalline semiconductorfilm by a method different from the ones described in Embodiments 1through 4. The only difference between this embodiment and Embodiment 1is in the step of forming the heat retaining film 13 and the subsequentsteps are identical. Therefore explanations for the identical steps areomitted here.

First, a substrate is prepared as in Embodiment 1. The substrate,denoted by 11, may be a glass substrate. Examples of the glass substrateinclude a synthesized quartz glass substrate, and a non-alkaline glasssubstrate such as a barium borosilicate substrate or analuminoborosilicate glass substrate. Transparent films such as PC(polycarbonate), PAr (polyarylate), PES (polyether sulfon) and PET(polyethylene telephthalate) may be used instead. For example, CorningNo. 7059 glass or No. 1737 glass (product of Corning Incorporated) is apreferable material for the substrate 11.

A base insulating film 12 is formed on the substrate 11 by a knownmethod (LPCVD, plasma CVD, or the like) from a silicon nitride film, asilicon oxynitride film, a silicon oxide film or a like other film. Inthis embodiment, a silicon oxynitride film (composition ratio: Si=32%,O=27%, N=24%, H=17%) is formed to a thickness of 50 nm.

On the base insulating film 12, a porous silicon film is formed as theheat retaining film 13. The porous silicon film is formed by, forexample, adding an iodine solution to an SOG solution through spincoating, drying the mixture to separate iodine, and subjecting it toheat treatment at a temperature of about 400° C. In this embodiment, theporous silicon film has a thickness of 50 nm.

Considering the heat conductivity of the substrate (1.4 W/m·k, in thecase of a quartz substrate) and the heat conductivity of silicon oxide(1 to 2 W/m·k), the heat retaining film 13 desirably has a heatconductivity of 1.0 W/m·k or less, more desirably 0.3 W/m·k or less.

After the heat retaining film 13 is formed, photolithography is used toform a resist mask and to etch unnecessary portions of the heatretaining film 13 away. A heat retaining film 14 is thus formed. Inetching the heat retaining film 13, dry etching that uses fluorine-basedgas or wet etching that uses a fluorine-based solution may be employed.When the wet etching is chosen, for example, the etchant may be amixture of 7.13% of ammonium hydrogen fluoride (NH₄HF₂) and 15.4% ofammonium fluoride (NH₄F). (The mixture is commercially available by thetrade name of LAL500 from Stella Chemipha Inc.)

When the heat retaining film 14 is a porous silicon film, the heatretaining film 14 has about 10¹¹ pores per centimeters square on itssurface. In order to level the surface of the heat retaining film 14, afirst insulating film 15 is formed using a silicon nitride film, asilicon oxynitride film, a silicon oxide film or the like by a knownmethod.

The subsequent steps follow the corresponding steps of Embodiment 1 toobtain a crystalline semiconductor film shown in FIG. 2C. The obtainedcrystalline semiconductor film has a region 26 in which crystal grainshaving a large grain size are formed as shown in the top view of FIG.2C. If the region 26 is used for a channel formation region of a TFT,the obtained TFT can have improved electric characteristics.

Embodiment 6

This embodiment shows an example of forming a crystalline semiconductorfilm by a method different from the ones described in Embodiments 1through 5. The only difference between this embodiment and Embodiment 1is in the step of forming the heat retaining film 13 and the subsequentsteps are identical. Therefore explanations for the identical steps areomitted here.

First, a substrate is prepared as in Embodiment 1. The substrate,denoted by 11, may be a glass substrate. Examples of the glass substrateinclude a synthesized quartz glass substrate, and a non-alkaline glasssubstrate such as a barium borosilicate substrate or analuminoborosilicate glass substrate. Transparent films such as PC(polycarbonate), PAr (polyarylate), PES (polyether sulfon) and PET(polyethylene telephthalate) may be used instead. For example, CorningNo. 7059 glass or No. 1737 glass (product of Corning Incorporated) is apreferable material for the substrate 11.

A base insulating film 12 is formed on the substrate 11 by a knownmethod (LPCVD, plasma CVD, or the like) from a silicon nitride film, asilicon oxynitride film, a silicon oxide film or a like other film. Inthis embodiment, a silicon oxynitride film (composition ratio: Si=32%,O=27%, N=24%, H=17%) is formed to a thickness of 50 nm.

On the base insulating film 12, a porous silicon oxide film is formed asthe heat retaining film 13. The porous silicon oxide film can readily beformed by anodizing a silicon substrate. The silicon substrate may beformed from semiconductor grade silicon such as CZ silicon or FZ siliconbut not limited thereto. It may be a solar battery grade (SOG grade)silicon substrate. The silicon substrate may be replaced by a glasssubstrate or a quartz substrate on which a silicon film is formed.Anodization is carried by mixing hydrofluoric acid (HF) and ethanol inequal parts to prepare an anodization solution and setting the currentdensity to 1 to 200 mA/cm². The thickness of the porous silicon oxidefilm is 1 to 5 μm. In this way, the heat retaining film 13 is formed onthe substrate from a porous silicon oxide film.

Considering the heat conductivity of the substrate (1.4 W/m·k, in thecase of a quartz substrate) and the heat conductivity of silicon oxide(1 to 2 W/m·k), the heat retaining film 13 desirably has a heatconductivity of 1.0 W/m·k or less, more desirably 0.3 W/m·k or less.

After the heat retaining film 13 is formed, photolithography is used toform a resist mask and to etch unnecessary portions of the heatretaining film 13 away. A heat retaining film 14 is thus formed. Inetching the heat retaining film 13, dry etching that uses fluorine-basedgas or wet etching that uses a fluorine-based solution may be employed.When the wet etching is chosen, for example, the etchant may be amixture of 7.13% of ammonium hydrogen fluoride (NH₄HF₂) and 15.4% ofammonium fluoride (NH₄F). (The mixture is commercially available by thetrade name of LAL500 from Stella Chemipha Inc.)

When the heat retaining film 14 is a porous silicon oxide film, the heatretaining film 14 has about 10¹¹ pores per centimeters square on itssurface. In order to level the surface of the heat retaining film 14, afirst insulating film 15 is formed using a silicon nitride film, asilicon oxynitride film, a silicon oxide film or the like by a knownmethod.

The subsequent steps follow the corresponding steps of Embodiment 1 toobtain a crystalline semiconductor film shown in FIG. 2C. The obtainedcrystalline semiconductor film has a region 26 in which crystal grainshaving a large grain size are formed as shown in the top view of FIG.2C. If the region 26 is used for a channel formation region of a TFT,the obtained TFT can have improved electric characteristics.

Embodiment 7

This embodiment shows an example of forming a crystalline semiconductorfilm by a method different from the ones described in Embodiments 1through 6. The only difference between this embodiment and Embodiment 1is in the step of forming the second insulating film 18 and thepreceding steps are identical. Therefore explanations for the identicalsteps are omitted here.

Following Embodiment 1, the process is finished up through the step offorming the semiconductor film 17.

Then the second insulating film 18 is formed on the semiconductor film17 in order to prevent impurities in a reflective film to be formedlater from diffusing into the semiconductor film. To make the secondinsulating film 18 function also as a reflection preventive film, thesecond insulating film has to have the optimal thickness. The optimalthickness varies depending on the wavelength of the laser beam as shownin FIGS. 3A and 3B. The second insulating film 18 is formed using asilicon nitride film, a silicon oxynitride film, a silicon oxide film ora like other film by a known method (LPCVD, plasma CVD or the like). Inthis embodiment, a silicon oxide film with a thickness of 45 nm isformed by plasma CVD.

On the second insulating film 18, a reflective film 19 is formed. If thereflective film 19 is a metal film, the film is formed by a known methodsuch as sputtering or evaporation to a thickness of 10 to 200 nm(preferably 10 to 100 nm). The metal film may be formed of an elementselected from the group consisting of Ta, W, Ti, Mo, Al, Cu, Cr and Nd,or of an alloy material, or compound material, containing the aboveelements as its main ingredient. An Ag—Pd—Cu alloy may also be used. Inthis embodiment, a Cr film is formed to a thickness of 50 nm.

After forming the reflective film 19, a resist mask is formed andunnecessary portions of the reflective film 19 is etched away byphotolithography. A reflective film 20 is thus formed. The shape of thereflective film 20 is not particularly limited but desirably ispolygonal in top view with one or more vertex of the polygon beingsmaller than 60°. The vertex desirably coincides with the end of theheat retaining film through the first insulating film, the semiconductorfilm and the second insulating film. The vertex smaller than 60° willhereinafter be called a vertex A. With the reflective film shaped assuch, crystal nuclei are generated at a smaller density in thesemiconductor film below a region around the vertex A while thesemiconductor film that has been irradiated with a laser beam coolsdown. Thus collision between growing crystal grains can be avoided (FIG.7A).

FIG. 7B is a diagram illustrating a crystallization step in which thesubstrate is irradiated with a laser beam from the front side. Incrystallization by laser annealing, hydrogen contained in thesemiconductor film is desirably released before the annealing.Appropriately, the semiconductor film is exposed in nitrogen atmosphereat 400 to 500° C. for about an hour to reduce the hydrogen content to 5atom % or less. This improves the resistance of the film against laserremarkably.

A description is given of a laser oscillator used in laser annealing. Anexcimer laser is high power and currently can generate a high frequencypulse on the order of 300 Hz, and hence it is often used in laserannealing. Other than the pulse oscillation excimer laser, a continuousoscillation excimer laser, an Ar laser, a YAG laser, a YVO₄ laser, a YLFlaser, etc. may be used. The laser beam irradiation can be carried outin vacuum, atmospheric air, nitrogen atmosphere, or other types ofatmospheres. The substrate may be heated up to about 500° C. before thelaser beam irradiation. This will lower the heat loss rate in thesemiconductor film to increase the grain size of the crystal grains.

One of the laser oscillators listed above is chosen to irradiate thesubstrate from the front side in one of the above atmospheres, wherebythe semiconductor film is crystallized.

Here, setting the ends of the reflective film as the borders, a regionincluding the heat retaining film 14 is designated as a region A, aregion including the reflective film 20 is designated as a region B, anda region that does not include the heat retaining film 14 nor thereflective film 20 is designated as a region C. (See FIGS. 8A and 8B.)

When irradiated with a laser beam, the semiconductor film is melted.However, the effective irradiation intensity of laser beam on thesemiconductor film in the region B is lower than on the semiconductorfilm in the region A and the region C because the semiconductor film inthe region B is covered with the reflective film, which reflects thelaser beam. Accordingly a solid phase semiconductor region 33 is leftbelow the reflective film, and crystal growth begins immediately afterthe laser beam irradiation from the solid phase semiconductor region 33following the temperature gradient created in the semiconductor film. Onthe other hand, the irradiation intensity of laser beam is strong in theregions A and C owing to the effect of the reflection preventive film.The density of generated crystal nuclei 34 is particularly low in thesolid phase semiconductor region 33 in the vicinity of the vertex A, forthe vertex A has a small angle of less than 60°. Furthermore, thesemiconductor film remains melted for a long time in the region A due tothe presence of the heat retaining film 14. Therefore the crystal nuclei34 grow toward the region A. Thus crystal grains having a large grainsize are formed in the semiconductor film in the region A. In the regionC, the semiconductor film does not have the heat retaining film 14underneath and hence cools faster than in the region A to generatecrystal nuclei and start the crystal growth.

Thus a crystalline semiconductor film 35 is formed through the laserbeam irradiation. The crystalline semiconductor film 35 is heated at 300to 450° C. in an atmosphere containing 3 to 100% of hydrogen, or heatedat 200 to 450° C. in an atmosphere containing hydrogen that is generatedby plasma. The heat treatment reduces the remaining defects.

The reflective film is then removed by photolithography or othermethods, and then the second insulating film is removed byphotolithography or other methods.

The crystalline semiconductor film 35 formed in this way has a region 36in which crystal grains having a large grain size are formed as shown inthe top view of FIG. 8B. If the region 36 is used for a channelformation region of a TFT, the obtained TFT can have improved electriccharacteristics.

This embodiment may be combined freely with one of Embodiments 1 through6.

Embodiment 8

The manufacturing method of the pixel portion and TFT (n-channel typeTFT and p-channel type TFT) of the driver circuit provided at theperiphery of the pixel portion simultaneously on the same substrate isexplained in detail using FIGS. 9 to 12. In this specification, thesubstrate on which is formed the driver circuit, the pixel TFT andretention capacitor is referred to as an active matrix substrate as amatter of convenience.

The crystalline semiconductor film shown in FIG. 9A can be obtained bywhichever method among Embodiments 1 to 7. In this embodiment, themanufacturing method of TFT is explained by corresponding the crosssectional view of FIG. 9A and the cross sectional view taken along thedashed line of A-A′ of FIG. 2C or FIG. 8B. It is also possible that TFTis formed by using the cross-sectional view which is used when thecrystalline semiconductor film is formed in Embodiments 1 to 7. In FIG.9A, the reference numeral 101 a to 101 f are heat insulating films, andreference numeral 102 a to 102 f are insulating films for preventingdiffusion of impurity element from the heat insulating film.

First, above mentioned crystalline semiconductor film is patterned indesired shape to obtain the semiconductor films 103 a to 103 f. In thisembodiment, above mentioned crystalline semiconductor film is subjectedto a patterning process using a photolithography method, to obtain thesemiconductor films 103 a to 103 f.

Further, after the formation of the semiconductor films 103 a to 103 f,a minute amount of impurity element (boron or phosphorus) may be dopedto control a threshold value of the TFT.

A gate insulating film 107 is then formed for covering the semiconductorfilms 103 a to 103 f. The gate insulating film 107 is formed of aninsulating film containing silicon by a plasma CVD method or asputtering method into a film thickness of from 40 to 150 nm. In thisembodiment, the gate insulating film 107 is formed of a silicon nitrideoxide film into a thickness of 110 nm by a plasma CVD method(composition ratio Si=32%, O=59%, N=7%, and H=2%). Of course, the gateinsulating film is not limited to the silicon nitride oxide film, and another insulating film containing silicon may be used as a single layeror a lamination structure.

Besides, when the silicon oxide film is used, it can be possible to beformed by a plasma CVD method in which TEOS (tetraethyl orthosilicate)and O₂ are mixed and discharged at a high frequency (13.56 MHZ) electricpower density of 0.5 to 0.8 W/cm² with a reaction pressure of 40 Pa anda substrate temperature of 300 to 400° C. Good characteristics as thegate insulating film can be obtained in the manufactured silicon oxidefilm thus by subsequent thermal annealing at 400 to 500° C.

Then, as shown in FIG. 9A, on the gate insulating film 107, a firstconductive film 108 with a thickness of 20 to 100 nm and a secondconductive film 109 with a thickness of 100 to 400 nm are formed andlaminated. In this embodiment, the first conductive film 108 of TaN filmwith a film thickness of 30 nm and the second conductive film 109 of a Wfilm with a film thickness of 370 nm are formed into lamination. The TaNfilm is formed by sputtering method with a Ta target under a nitrogencontaining atmosphere. Besides, the W film is formed by the sputteringmethod with a W target. The W film may be formed by a thermal CVD methodusing tungsten hexafluoride (WF₆). Whichever method is used, it isnecessary to make the material have low resistance for use as the gateelectrode, and it is preferred that the resistivity of the W film is setto less than or equal to 20 μΩcm. By making the crystal grains large, itis possible to make the W film have lower resistivity. However, in thecase where many impurity elements such as oxygen are contained withinthe W film, crystallization is inhibited and the resistance becomeshigher. Therefore, in this embodiment, by forming the W film by asputtering method using a tungsten target with a high purity of99.9999%, and in addition, by taking sufficient consideration to preventimpurities within the, gas phase from mixing therein during the filmformation, a resistivity of from 9 to 20 μΩcm can be realized.

Note that, in this embodiment, the first conductive film 108 is made ofTaN, and the second conductive film 109 is made of W, but the materialis not particularly limited thereto, and either film may be formed of anelement selected from the group consisting of Ta, W, Ti, Mo, Al, Cu, Cr,and Nd, or an alloy material or a compound material containing the aboveelement as its main constituent. Besides, a semiconductor film, typifiedby a polycrystalline silicon film doped with an impurity element such asphosphorus, may be used. Further, an AgPdCu alloy may be used. Besides,any combination may be employed such as a combination in which the firstconductive film is formed of tantalum (Ta) and the second conductivefilm is formed of W, a combination in which the first conductive film isformed of titanium nitride (TiN) and the second conductive film isformed of W, a combination in which the first conductive film is formedof tantalum nitride (TaN) and the second conductive film is formed ofAl, or a combination in which the first conductive film is formed oftantalum nitride (TaN) and the second conductive film is formed of Cu.

Next, masks 110 to 115 made of resist are formed using aphotolithography method, and a first etching process is performed inorder to form electrodes and wirings. This first etching process isperformed with the first and second etching conditions. In Thisembodiment, as the first etching conditions, an ICP (inductively coupledplasma) etching method is used, a gas mixture of CF₄, Cl₂ and O₂ is usedas an etching gas, the gas flow rate is set to 25/25/10 sccm, and plasmais generated by applying a 500 W RF (13.56 MHz) electric power to a coilshape electrode under 1 Pa. A dry etching device with ICP (ModelE645-□ICP) produced by Matsushita Electric Industrial Co. Ltd. is usedhere. A 150 W RF (13.56 MHz) electric power is also applied to thesubstrate side (test piece stage) to effectively apply a negativeself-bias voltage. The W film is etched with the first etchingconditions, and the end portion of the first conductive layer is formedinto a tapered shape.

Thereafter, the first etching conditions are changed into the secondetching conditions without removing the masks 110 to 115 made of resist,a mixed gas of CF₄ and Cl₂ is used as an etching gas, the gas flow rateis set to 30/30 sccm, and plasma is generated by applying a 500 W RF(13.56 MHz) power to a coil shape electrode under 1 Pa to therebyperform etching for about 30 seconds. A 20 W RF (13.56 MHz) electricpower is also applied to the substrate side (test piece stage) toeffectively a negative self-bias voltage. The W film and the TaN filmare both etched on the same order with the second etching conditions inwhich CF₄ and Cl₂ are mixed. Note that, the etching time may beincreased by approximately 10 to 20% in order to perform etching withoutany residue on the gate insulating film.

In the first etching process, the end portions of the first and secondconductive layers are formed to have a tapered shape due to the effectof the bias voltage applied to the substrate side by adopting masks ofresist with a suitable shape. The angle of the tapered portions may beset to 15° to 45°. Thus, first shape conductive layers 117 to 122 (firstconductive layers 117 a to 122 a and second conductive layers 117 b to122 b) constituted of the first conductive layers and the secondconductive layers are formed by the first etching process. Referencenumeral 116 denotes a gate insulating film, and regions of the gateinsulating film which are not covered by the first shape conductivelayers 117 to 122 are made thinner by approximately 20 to 50 nm byetching.

Then, a first doping process is performed to add an impurity element forimparting an n-type conductivity to the semiconductor layer withoutremoving the mask made of resist (FIG. 9B). Doping may be carried out byan ion doping method or an ion injecting method. The condition of theion doping method is that a dosage is 1×10¹³ to 5×10¹⁵ atoms/cm², and anacceleration voltage is 60 to 100 keV. In this embodiment, the dosage is1.5×10¹⁵ atoms/cm² and the acceleration voltage is 80 keV. As theimpurity element for imparting the n-type conductivity, an element whichbelongs to group 15 of the periodic table, typically phosphorus (P) orarsenic (As) is used, and phosphorus is used here. In this case, theconductive layers 117 to 122 become masks to the impurity element forimparting the n-type conductivity, and high concentration impurityregions 123 to 127 are formed in a self-aligning manner. The impurityelement for imparting the n-type conductivity is added to the highconcentration impurity regions 123 to 127 in the concentration range of1×10²⁰ to 1×10²¹ atoms/cm³.

Next, the second etching process is carried out without removing themask comprising a resist. CF₄ and Cl₂ and O₂ are used for an etching gasand the W film is selectively etched. At this occasion, there are formedsecond conductive layers 128 b through 133 b by the second etchingprocess. Meanwhile, the first conductive layers 117 a through 122 a arehardly etched and first conductive layers 128 a through 133 a areformed. Next, by carrying out a second doping process, a state of FIG.9C is provided. In doping, the second conductive layers 128 b through133 b are used as masks against an impurity element and the doping iscarried out such that the impurity element is added to semiconductorlayers on lower sides of taper portions of first conductive layers. Inthis way, there are formed impurity regions 134 through 138 overlappingthe first conductive layers. A concentration of phosphorus (P) added tothe impurity region is provided with a gradual concentration gradient inaccordance with a film thickness of the taper portion of the firstconductive layer. Further, in the semiconductor layer overlapping thetaper portion of the first conductive layer, from an end portion of thetaper portion of the first conductive layer toward an inner side, theimpurity concentration is more or less reduced, however, theconcentration stays to be substantially the same degree. Further, thefirst impurity regions 123 through 127 are also added with the impurityelement to thereby form impurity regions 139 through 143.

Next, the third etching process is carried out without removing the maskcomprising a resist. The third etching process is carried out forpartially etching a taper portion of the first conductive layer andreducing a region overlapping the semiconductor layer. The third etchingis carried out by using CHF₃ for an etching gas and using a reactive ionetching process (RIE process). And also the third etching process can beusing an ICP process. By the third etching, there are formed firstconductive layers 144 through 149. At this occasion, the insulating film116 is simultaneously etched and there is formed an insulating film 150and 151.

By the third etching, there are formed impurity regions (LDD regions)134 a through 138 a not overlapping the first conductive layers 144through 148. Further, impurity regions (GOLD region) 134 b through 138 bstay to overlap the first conductive layers 144 through 148.

Thereby, according to the embodiment, in Embodiment 8, a differencebetween the impurity concentration at the impurity regions (GOLD region)134 b through 138 b overlapping the first conductive layers 144 through148 and the impurity concentration at the impurity regions (LDD regions)134 a through 138 a not overlapping the first conductive layers 144through 148, can be reduced, and a reliability can be promoted.

Next, after removing the mask comprising a resist, there are formedmasks 152 through 154 comprising resists are newly formed and a thirddoping process is carried out. By the third doping process, there areformed impurity regions 155 through 160 added with an impurity elementfor providing a conductive type reverse to the conductive type,mentioned above, to the semiconductor layer for constituting anactivation layer of a p-channel type TFT. The first conductive layers128 a through 132 a are used as masks against the impurity element andthe impurity element for providing p-type is added to thereby form theimpurity regions in a self-aligning manner. According to the embodiment,the impurity regions 155 through 160 are formed by an ion doping processusing diborane (B₂H₆). In the third doping process, the semiconductorlayer for forming the n-channel type TFT is covered by the masks 152through 154 comprising resists. Although the impurity regions 155through 160 are respectively added with phosphorus by differentconcentrations by the first doping process and the second dopingprocess, by carrying out the doping process in any of the regions suchthat the concentration of the impurity element for providing p-typebecomes 2×10²⁰ through 2×10²¹ atoms/cm³, the regions function as thesource region and the drain region of the p-channel type TFT andaccordingly, no problem is posed. According to the embodiment, a portionof the semiconductor layer for constituting the activation layer of thep-channel type TFT is exposed and therefore, there is an advantage thatthe impurity element (boron) is easier to add than in Embodiment 8.

By the above-described steps, the respective semiconductor layers areformed with the impurity regions.

Next, the masks 152 through 154 comprising resists are removed and afirst interlayer insulating film 161 is formed. The first interlayerinsulating film 161 is formed by an insulating film including siliconhaving the thickness of 100 through 200 nm by using a plasma CVD processor a sputtering process. According to the embodiment, a siliconoxynitride film having a film thickness of 150 nm is formed by a plasmaCVD process. Naturally, the first interlayer insulating film 161 is notlimited to the silicon oxynitride film but other insulating filmincluding silicon may be used as a single layer or a laminatedstructure.

Next, as shown in FIG. 10C, there is carried out a step of activatingthe impurity elements added to the respective semiconductor layers. Theactivating step is carried out by a thermal annealing process using afurnace annealing furnace. The thermal annealing process may be carriedout at 400 through 700° C., representatively, 500 through 550° C. in anitrogen atmosphere having an oxygen concentration equal to or smallerthan 1 ppm, preferably, equal to or smaller than 0.1 ppm and accordingto the embodiment, the activating process is carried out by a heattreatment at 550° C. for 4 hours. Further, other than the thermalannealing process, a laser annealing process or a rapid thermalannealing process (RTA process) is applicable.

Further, according to the embodiment, simultaneously with the activatingprocess, nickel used as a catalyst in crystallization is gettered by theimpurity regions 139, 141, 142, 155 and 158 including phosphorus at ahigh concentration and a nickel concentration in the semiconductor layerfor mainly constituting a channel forming region is reduced. Accordingto TFT having the channel forming region fabricated in this way, the offcurrent value is reduced, crystallizing performance is excellent andaccordingly, high electric field effect mobility is provided andexcellent electric characteristic can be achieved.

Further, the activating process may be carried out prior to forming thefirst interlayer insulating film. However, when a used wiring materialis weak at heat, it is preferable to carry out the activating processafter forming the interlayer insulating film (insulating film having amajor component of silicon, for example, silicon nitride film) forprotecting the wiring as in the embodiment.

Further, there is carried out a step of hydrogenating the semiconductorlayer by carrying out heat treatment at 300 through 550° C. for 1through 12 hours in an atmosphere including 3 through 100% of hydrogen.According to the embodiment, there is carried out a heat treatment at410° C. for 1 hour in a nitrogen atmosphere including about 3% ofhydrogen. This step is the step of terminating dangling bond of thesemiconductor layer by hydrogen included in the interlayer insulatingfilm. As other means of hydrogenation, plasma hydrogenation (usinghydrogen excited by plasma) may be carried out.

Further, when a laser annealing process is used as the activatingprocess, after carrying out the hydrogenation, it is preferable toirradiate laser beam such as excimer laser or YAG laser.

A second interlayer insulating film 162 made from an inorganicinsulating material or from an organic insulating material is formednext on the first interlayer insulating film 161. An acrylic resin filmhaving a film thickness of 1.6 μm is formed in embodiment 8, and thematerial used may have a viscosity from 10 to 1000 cp, preferablybetween 40 and 200 cp. A material in which unevenness is formed on itssurface is used. Further a film having a level surface may also be usedas the second interlayer insulating film 162.

In order to prevent specular reflection, the surface of a pixelelectrode is made uneven by forming the second interlayer insulatingfilm from a material which forms an uneven surface in embodiment 8.Further, the electrode surface can be made to be uneven and have lightscattering characteristics, and therefore a convex portion may also beformed in a region below the pixel electrode. The formation of theconvex portion can be performed by the same photomask as that forforming the TFTs, and therefore it can be formed without increasing thenumber of process steps. Note that the convex portion may also be formedsuitably on the substrate of pixel portion region outside of the wiringsand TFTs. Unevenness is formed in the surface of the pixel electrodealong the unevenness formed in the surface of the insulating film whichcovers the convex portion.

Wirings 163 to 167 for electrically connecting the various impurityregions are then formed in a driver circuit in order. Note that alamination film of a 50 nm thick Ti film and a 500 nm thick alloy film(an alloy of Al and Ti) is patterned for forming the wirings.

Furthermore, a pixel electrode 170, a gate wiring 169, and a connectionelectrode 168 are formed in a pixel portion. (See FIG. 11.) Anelectrical connection is formed with the pixel TFT and the source wiring(lamination of the impurity regions 133 b and 149) by the connectionelectrode 168. Further, the gate wiring 169 forms an electricalconnection with the gate electrode of the pixel TFT. The pixel electrode170 forms an electrical connection with the drain region of the pixelTFT, and in addition, forms an electrical connection with thesemiconductor layer 158 which functions as one electrode forming thestorage capacitor. It is preferable to use a material having superiorreflectivity, such as a film having Al or Ag as its main constituent, ora lamination film of such films, as the pixel electrode 170.

A CMOS circuit composed of an n-channel TFT 501 and a p-channel TFT 502,a driver circuit 506 having an n-channel TFT 503, and the pixel portionhaving a pixel TFT 504 and a storage capacitor 505 can thus be formed onthe same substrate. The active matrix substrate is thus completed.

The n-channel TFT 501 of the driver circuit 506 has: a channel formingregion 171; the low concentration impurity region 134 b (GOLD region)which overlaps with the first conductive layer 144 that structures aportion of the gate electrode; the low concentration impurity region 134a (LDD region) formed on the outside of the gate electrode; and the highconcentration impurity region 139 which functions as a source region ora drain region. The p-channel TFT 502, which forms the CMOS circuit withthe n-channel TFT 501 by an electrical connection through the electrode166, has: a channel forming region 172; the impurity region 157 whichoverlaps with the gate electrode; the impurity region 156 which isformed on the outside of the gate electrode; and the high concentrationimpurity region 155 which functions as a source region or a drainregion. Further, the n-channel TFT 503 has: a channel forming region173; the low concentration impurity region 136 b (GOLD region) whichoverlaps with the first conductive layer 146 that structures a portionof the gate electrode; the low concentration impurity region 136 a (LDDregion) which is formed on the outside of the gate electrode; and thehigh concentration impurity region 141 which functions as a sourceregion or a drain region.

The pixel TFT 504 of the pixel portion has: a channel forming region174; the low concentration impurity region 137 b (GOLD region) whichoverlaps with the first conductive layer 147 that structures a portionof the gate electrode; the low concentration impurity region 137 a (LDDregion) formed on the outside of the gate electrode; and the highconcentration impurity region 142 which functions as a source region ora drain region. Further, impurity element imparting a p-typeconductivity is added to the semiconductor layers 158 to 160 whichfunction as one electrode of the storage capacitor 505. The storagecapacitor 505 is formed by an electrode (lamination of the conductivelayer 148 and the region 132 b) and the semiconductor layers 158 to 160,with the insulating film 151 functioning as a dielectric.

The edge portions of the pixel electrodes are arranged so as to overlapthe source wirings such that gaps between the pixel electrodes areshielded without using a black matrix with the pixel structure ofembodiment 8.

A top surface diagram of the pixel portion of the active matrixsubstrate manufactured by embodiment 8 is shown in FIG. 12. Note thatportions corresponding to those of FIGS. 9 to 11 use the same referencenumerals. The dashed line B-B′ of FIG. 12 corresponds to a crosssectional diagram of FIG. 11 cut along the dashed line B-B′, and thedashed line C-C′ of FIG. 12 corresponds to a cross sectional diagram ofFIG. 11 cut along the dashed line C-C′.

The number of photomasks required to manufacture the active matrixsubstrate can be set to five in accordance with the processes shown byembodiment 8. As a result, the number of process steps can be reduced,and this can contribute to a lowering of the manufacturing cost andincreased yield ratio.

A structure such as that above optimizes the structure of the pixel TFTand TFTs composing each circuits of the driver circuit in response tothe specifications required, and it is possible to increase theoperating performance and the reliability of the semiconductor device.In addition, by forming the gate electrode using a conductive materialhaving heat resistance, the LDD regions, and source regions and drainregions are easily activated. Moreover, the wiring resistance can besufficiently lowered by forming the gate electrode using a gate wiringlow resistance material.

Embodiment 9

In this embodiment, a manufacturing process of a reflection type liquidcrystal display device from the active matrix substrate manufactured inaccordance with Embodiment 8 will be described hereinbelow. FIG. 13 isused for an explanation thereof.

First, in accordance with Embodiment 8, an active matrix substrate in astate shown in FIG. 11 is obtained, and thereafter, an orientation film171 is formed on the active matrix substrate of FIG. 11, at least on thepixel electrode 170, and is subjected to a rubbing process. Note that,in this embodiment, before the formation of the orientation film 171, aspacer (not illustrated) for maintaining a gap between the substrates isformed at a desired position by patterning an organic film such as anacrylic resin film. Further, spherical spacers may be scattered on theentire surface of the substrate in place of the columnar like spacer.

Next, an opposing substrate 171 is prepared. The colored layers 172, 173and a leveling film 174 are formed on the opposing substrate 171. Thered-colored layer 172 and the blue-colored layer 173 are partiallyoverlapped with each other, thereby forming a light shielding portion.Note that, the red-colored layer and a green-colored layer are partiallyoverlapped with each other, thereby forming a light shielding portion.

In this embodiment, the substrate shown in Embodiment 8 is used.Accordingly, in FIG. 12 showing a top view of the pixel portion inaccordance with Embodiment 8, light shielding must be performed at leastgaps between the gate wiring 169 and the pixel electrodes 170, a gapbetween the gate wiring 169 and the connection electrode 168, and a gapbetween the connection electrode 168 and the pixel electrode 170. Inthis embodiment, the opposing substrate and the active matrix substrateare stuck so that the light shielding portions from laminated layer ofcolored layer each other overlap with the positions which need to beshielded from light.

Like this, without using a black mask, the gaps between the respectivepixels are shielded from light by the light shielding portion. As aresult, the reduction of the manufacturing steps can be attained.

Next, the opposing electrode 175 from transparent conductive film isformed on the leveling film 174, at least on the pixel portion. Theorientation film 176 on the entire surface of the opposing substrate andthe rubbing process is performed.

Then, an active matrix substrate on which a pixel portion and a drivercircuit are formed is stuck with the opposing substrate by a sealingagent 177. In the sealing agent 177, a filler is mixed, and the twosubstrates are stuck with each other while keeping a uniform gap by theeffect of this filler and the columnar spacer. Thereafter, a liquidcrystal material 178 is injected between both the substrates toencapsulate the substrates completely by an encapsulant (notillustrated). A known liquid crystal material may be used as the liquidcrystal material 178. Thus, the reflection type liquid crystal displaydevice shown in FIG. 13 is completed. Then, if necessary, the activematrix substrate or the opposing substrate may be parted into desiredshapes. Further, a polarizing plate are adhered to only the opposingsubstrate (not illustrated). Then, an FPC is adhered using a knowntechnique.

The liquid crystal display device manufactured according to abovementioned way can be used as a display portion of various electronicdevice.

Embodiment 10

CMOS circuits and pixel portions formed in accordance with the presentinvention can be used in various electro-optical devices (active matrixtype liquid crystal display, active matrix type EC display and activematrix type EL display). In other words, the present invention can beapplied to all of the electronic equipments having these electro-opticaldevices as the display section.

The following can be given as examples of the electronic equipment:video cameras; digital cameras; projectors (rear type or front type);head mounted displays (goggle type display); car navigation systems; carstereo; personal computers; portable information terminals (such asmobile computers, portable telephones and electronic notebook). Anexample of these is shown in FIGS. 14, 15 and 16.

FIG. 14A shows a personal computer, and it includes a main body 3001, animage input section 3002, a display portion 3003, and a keyboard 3004.The present invention is applicable to the image input section 3002, thedisplay portion 3003, and other signal controlling circuits.

FIG. 14B shows a video camera, and it includes a main body 3101, adisplay portion 3102, a voice input section 3103, operation switches3104, a battery 3105, and an image receiving section 3106. The presentinvention is applicable to the display portion 3102 and other signalcontrolling circuits.

FIG. 14C shows a mobile computer, and it includes a main body 3201, acamera section 3202, an image receiving section 3203, operation switches3204, and a display portion 3205. The present invention is applicable tothe display portion 3205 and other signal controlling circuits.

FIG. 14D shows a goggle type display, and it includes a main body 3301;a display portion 3302; and an arm section 3303. The present inventionis applicable to the display portion 3302 and other signal controllingcircuits.

FIG. 14E shows a player using a recording medium which records a program(hereinafter referred to as a recording medium), and it includes a mainbody 3401; a display portion 3402; a speaker section 3403; a recordingmedium 3404; and operation switches 3405. This player uses DVD (digitalversatile disc), CD, etc. for the recording medium, and can be used formusic appreciation, film appreciation, games and Internet. The presentinvention is applicable to the display portion 3402 and other signalcontrolling circuits.

FIG. 14F shows a digital camera, and it includes a main body 3501; adisplay portion 3502; a view finder 3503; operation switches 3504; andan image receiving section (not shown in the figure). The presentinvention can be applied to the display portion 3502 and other signalcontrolling circuits.

FIG. 15A is a front-type projector, and it includes a projection device3601 and a screen 3602. The present invention is applicable to a liquidcrystal display device 3808 which comprises one of the projection device3601 and other signal controlling circuits.

FIG. 15B is a rear-type projector, and it includes a main body 3701, aprojection device 3702, a mirror 3703, and a screen 3704. The presentinvention is applicable to a liquid crystal display device 3808 whichcomprises one of the projection device 3702 and other signal controllingcircuits.

FIG. 15C is a diagram showing an example of the structure of theprojection devices 3601, 3702 in FIGS. 15A and 15B. The projectiondevice 3601 or 3702 comprises a light source optical system 3801,mirrors 3802, 3804 to 3806, dichroic mirrors 3803, a prism 3807, liquidcrystal display devices 3808, phase difference plates 3809, and aprojection optical system 3810. The projection optical system 3810 iscomposed of an optical system including a projection lens. This exampleshows an example of three plate type but not particularly limitedthereto. For instance, the invention may be applied also to a singleplate type optical system. Further, in the light path indicated by anarrow in FIG. 15C, an optical system such as an optical lens, a filmhaving a polarization function, a film for adjusting a phase difference,and an IR film may be suitably provided by a person who carries out theinvention.

FIG. 15D is a diagram showing an example of the structure of the lightsource optical system 3801 in FIG. 15C. In this embodiment, the lightsource optical system 3801 comprises a reflector 3811, alight source3812, lens arrays 3813, 3814, a polarization conversion element 3815,and a condenser lens 3816. The light source optical system shown in FIG.15D is merely an example, and is not particularly limited to theillustrated structure. For example, a person who carries out theinvention is allowed to suitably add to the light source optical systeman optical system such as an optical lens, a film having a polarizationfunction, a film for adjusting a phase difference, and an IR film.

Note that a transmission electro-optical device is used as the projectorshown in FIG. 15, a reflection type electro-optical device is notillustrated.

FIG. 16A is a portable telephone, and it includes a main body 3901, anaudio output section 3902, an audio input section 3903, a displayportion 3904, operation switches 3905, and an antenna 3906. The presentinvention can be applied to the audio output portion 3902, the audioinput portion 3903, the display portion 3904, and other signal circuit.

FIG. 16B is a portable book (electronic book), and it includes a mainbody 4001, display portions 4002 and 4003, a recording medium 4004,operation switches 4005, and an antenna 4006. The present invention canbe applied to the display portions 4002 and 4003, and other signalcircuit.

FIG. 16C is a display, and it includes a main body 4101, a support stand4102, and a display portion 4103. The present invention can be appliedto the display portion 4103. The display of the present invention isadvantageous for a large size screen in particular, and is advantageousfor a display equal to or greater than 10 inches (especially equal to orgreater than 30 inches) in diagonal.

The applicable range of the present invention is thus extremely wide,and it is possible to apply the present invention to electronicequipment in all fields. Further, the electronic equipment of theembodiment 10 can be realized by using a constitution of any combinationof the embodiments 1 to 9.

The usefulness provided by the structure of the present invention isoutlined in the following.

(a) The structure is simple and applicable to existing process ofmanufacturing a TFT.

(b) There is no need for positioning technique of micron-level precisionto position a slit or the like. Nor, a special positioning unit isrequired in a laser irradiation apparatus. The invention can utilize anordinary laser irradiation apparatus without any modification.

(c) A TFT can be formed from a semiconductor film without leaving amaterial that has nothing to do with the function of the TFT in thesemiconductor film.

(d) The method according to the present invention is capable of formingcrystal grains having a large grain size at designed positions, on topof possessing all of the above advantages (a) through (c). When acrystalline semiconductor film having such crystal grains is used toform a TFT, the obtained TFT can have greatly improved electriccharacteristics.

1. A semiconductor device having a thin film transistor, comprising: a heat retaining film formed on an insulating surface; a semiconductor film formed in contact with said insulating surface and heat retaining film; and a channel formation region formed in said semiconductor film and in contact with said heat retaining film.
 2. A semiconductor device having a thin film transistor, comprising: a heat retaining film formed on an insulating surface; an insulating film covering said heat retaining film; a semiconductor film in contact with said insulating film and said insulating surface; a channel formation region formed in said semiconductor film and in contact with said heat retaining film via said insulating film therebetween.
 3. The semiconductor device according to claim 1 wherein the heat retaining film comprises silicon oxide, which contains one selected from the group consisting of methyl (CH₃) group, ethyl (C₂H₅) group, propyl (C₃H₇) group, butyl (C₄H₉) group, vinyl (C₂H₃) group, phenyl (C₆H₅) group, and CF₃ group.
 4. The semiconductor device according to claim 2 wherein the heat retaining film comprises silicon oxide, which contains one selected from the group consisting of methyl (CH₃) group, ethyl (C₂H₅) group, propyl (C₃H₇) group, butyl (C₄H₉) group, vinyl (C₂H₃) group, phenyl (C₆H₅) group, and CF₃ group.
 5. The semiconductor device according to claim 1 wherein said heat retaining film is selected from the group consisting of a porous silicon film and a porous silicon oxide film.
 6. The semiconductor device according to claim 2 wherein said heat retaining film is selected from the group consisting of a porous silicon film and a porous silicon oxide film.
 7. The semiconductor device according to claim 1 wherein said heat retaining film has a heat conductivity of 1.0 W/mk or less.
 8. The semiconductor device according to claim 2 wherein said heat retaining film has a heat conductivity of 1.0 W/mk or less.
 9. The semiconductor device according to claim 1 wherein said heat retaining film has a heat conductivity of 0.3 W/mk or less.
 10. The semiconductor device according to claim 2 wherein said heat retaining film has a heat conductivity of 0.3 W/mk or less.
 11. The semiconductor device according to claim 1 wherein said semiconductor device is selected from the group consisting of a mobile phone, a video camera, a digital camera, a projector, a goggle-type display, a personal computer, a DVD player, an electronic book, and a portable information terminal.
 12. The semiconductor device according to claim 2 wherein said semiconductor device is an electroluminescence display device. 